Network filter

ABSTRACT

The invention is directed to a network filter that comprises at least one phase input (L 1 , L 2 , L 3 ) and a protective conductor ( 1 ), whereby each phase input (L 1 , L 2 , L 3 ) is connected via a filter inductance ( 2 ) to a corresponding phase output (A 1 , A 2 , A 3 ), whereby the input side of each filter inductance ( 2 ) is connected to the protective conductor ( 1 ) via a filter capacitor ( 3 ), and whereby an auxiliary inductance ( 7 ) is circuited between each filter capacitor ( 3 ) and the protective conductor ( 1 ). What the auxiliary inductance ( 7 ) achieves is that the inductances in the filter can be kept small, as a result whereof they can be realized by simply slipping a ferrite core on over a conductor.

[0001] The invention is directed to a network filter that comprises at least one filter inductance and at least one filter capacitor.

[0002] Network filters of the species initially cited are known that comprise two through four phase inputs and a protective conductor or, respectively, a filter housing, whereby each phase input is connected to the corresponding phase output via a filter inductance. Capacitors can be connected to each side of the filter inductances. These capacitors can be circuited to the protective conductor or, respectively, to the filter housing either directly or via a star point that is connected to a further capacitor. The known network filters can be employed, for example, to screen a frequency converter connected to the network, i.e. to prevent the feed of high-frequency interference from the frequency converter into the power network. The network filter is connected between the power network and the frequency converter for this purpose

[0003] In order to meet standards defined in conjunction with the frequency converter, for example the class A limit value according to EN 55011, the filter must exhibit certain minimum attenuation properties. With respect to said standard, a limit value of 79 dB μV is defined for line-bound interference in the range between 0.15 and 0.5 MHz and a limit value of 73 dB μV (quasi peak value) is defined therefor between 0.5 and 30 MHz. In order to meet these specifications, it is necessary that the filter inductances employed in the known network filters exhibit a relatively high inductance.

[0004] In order to realize such filter inductances, what are referred to as current-compensated inductors with ferrite core are usually employed, these being wound such that the magnetic flux induced in the ferrite core by the main current (push-pull) yields a sum of approximately zero. The inductor windings can be composed of one or more turns. Correspondingly many turns must be wound in order to achieve high inductance values.

[0005] This manufacturing method has the disadvantage that it is time-consuming and costly and that, in particular, it places narrow limits in view of the mechanical realizability (weight and size) of network filters.

[0006] The employment of filter inductances that comprise lower values and that could therefore be produced by simply slipping a ferrite core over a conductor is not possible given the aforementioned network filters. When the filter inductances are too small, the attenuation properties of the network filter become poorer, so that the network filter no longer meets the corresponding standards in conjunction with, for example, a frequency converter connected thereto.

[0007] It is therefore an object of the present invention to specify a network filter that comprises good filter properties with low filter inductances.

[0008] This object is inventively achieved by a network filter according to claim 1. Advantageous developments of the invention can be derived from the other claims.

[0009] The invention specifies a network filter that comprises at least one phase input and a protective conductor. The protective conductor represents the high-frequency ground reference of the filter and can, for example, be produced by a filter housing. Each phase input is connected via a filter inductance to a corresponding phase output. At its input side, each filter inductance is connected via a filter capacitor to the protective conductor. Additionally, the network filter comprises at least one auxiliary inductance that is arranged such that an auxiliary inductance is circuited between each filter capacitor and the protective conductor.

[0010] The inventive network filter has the advantage that the asymmetrical screening effect of the filter is enhanced in the long-wave range (approximately 150 kHz) by the capacitors circuited to the protective conductor. Additionally, the auxiliary inductance effectively blocks higher-frequency ground currents (approximately 1 MHz) from skipping over onto the network input since the impedance of the auxiliary inductance increases with increasing frequency.

[0011] In an especially advantageous embodiment of the invention, the filter capacitors are connected to a common star point that is connected to the protective conductor via an auxiliary inductance. It is thereby possible to connect the star point to the auxiliary inductance via an auxiliary capacitor connected in series therewith, the filter properties being improved even further as a result thereof.

[0012] The filter inductances can be reduced due to the improved network filter properties. A network filter is therefore especially advantageous wherein the filter inductances amount to less than 50 μH. Such filter inductances can be produced with reduced outlay or even without outlay for winding a wire around a coil body.

[0013] Filter inductances <50 μH can be advantageously realized by a conductor that is surrounded by a magnetically conductive ring. Such filter inductances can be simply and inexpensively manufactured with little outlay and without winding a wire.

[0014] The magnetically conductive ring that surrounds the conductor for forming a filter inductance can, for example, be a ferrite core. Such ferrite cores are commercially obtainable, as a result whereof the manufacture of the filter inductances becomes even simpler and even less expensive.

[0015] Values <50 μH can be employed for the auxiliary inductance. Such a small auxiliary inductance suffices for filtering out high-frequency interference since the A. C. resistance of an inductance increases proportionally with the frequency. Auxiliary inductances having the aforementioned values are manufactured in a standard way a mass-produced goods and can therefore be very cost-beneficially acquired.

[0016] Further, a network filter is especially advantageous wherein the filter capacitors and the auxiliary capacitor amount to between 1 nF and 10 μF. These typical values can be easily realized with commercially obtainable capacitors and are excellently suited for meeting the class A limit value according to EN 55011.

[0017] The inventive network filter can also be especially advantageously configured as two-stage or multi-stage filter. To this end, one or more further filter inductances are connected in series with each filter inductance. Such a two-stage or multi-stage network filter exhibits even better filter properties compared to a single-stage filter.

[0018] The invention is explained in greater detail below on the basis of exemplary embodiments and the appertaining Figures.

[0019]FIG. 1 shows the schematic illustration of an inventive, single-phase network filter by way of example.

[0020]FIG. 2 shows the schematic illustration of an inventive three-phase network filter without neutral conductor by way of example.

[0021]FIG. 3 shows the schematic illustration of another inventive three-phase network filter without neutral conductor by way of example.

[0022]FIG. 4 shows the schematic illustration of a three-phase network filter with neutral conductor by way of example.

[0023]FIG. 1 shows an inventive network filter with a phase input L1, a phase output A1, a neutral conductor input N and a neutral conductor output N′. Inputs and outputs are respectively connected to one another by a filter inductance 2. The inputs L1, N can be connected to a power network, whereas the outputs A1, N′ can, for example, be connected to a frequency converter. The network filter also comprises a protective conductor 1.

[0024] Filter capacitors 3 that connect the filter inductances 2 to the protective conductor 1 are arranged at the input side of the filter inductances 2. An auxiliary inductance 7 is also respectively connected in series between the protective conductor 1 and the filter capacitors 2. Further capacitors 5 are circuited at the input side and output side of the filter inductances 2. The auxiliary inductance 7 prevents high-frequency interference occurring in the protective conductor 1 at the output side of the network filter from being supplied into the phase input L1 or into the neutral conductor input N and, thus, into the power network.

[0025]FIG. 2 shows an inventive network filter with three phase inputs L1, L2, L3 as well as three phase outputs A1, A2, A3. The filter also comprises a protective conductor 1. The filter capacitors 3 arranged at the input side of the filter inductance 2 are conducted to a common star point 4 of the input side. From there, they are connected to the protective conductor 1 via an auxiliary capacitor 8 and an auxiliary inductance 7 connected in series therewith. The auxiliary inductance 7 prevents high-frequency interference that occurs at the output side of the network filter in the protective conductor 1 from being supplied into the power network. The quality of the network filter is improved by the auxiliary capacitor 8, so that it manages with smaller filter inductances 2 given unaltered attenuation properties.

[0026]FIG. 3 shows a three-phase filter wherein the filter capacitors 3 are connected directly to the auxiliary inductance 7 or, respectively, the protective conductor 1 without a further auxiliary capacitor. The auxiliary inductor 7 prevents high-frequency interference that occurs at the output side of the network filter in the protective conductor 1 from being supplied into the power network.

[0027]FIG. 4 shows a three-phase network filter 1 wherein the filter capacitors 3 arranged at the input side of the filter inductances 2 are circuited to a common star point 4 at the input side that is connected to the neutral conductor input N. The input-side star point 4 is connected to the protective conductor 1 via an auxiliary capacitor 8 and an auxiliary inductance 7 connected in series therewith.

[0028] The filters shown in the Figures are single-stage filters that can be easily expanded into multi-stage filters with improved filter properties by multiple series connection of the circuit part identified with the broken-line marking.

[0029] The invention is not limited to the embodiment shown by way of example but is defined in its broadest form by claim 1. 

1. Network filter that comprises at least one phase input (L1, L2, L3) and a protective conductor (1), whereby each phase input (L1, L2, L3) is connected via a filter inductance (2) to a corresponding phase output (A1, A2, A3), whereby the input side of each filter inductance (2) is connected to the protective conductor (1) via a filter capacitor (3), and whereby an auxiliary inductance (7) is circuited between each filter capacitor (3) and the protective conductor (1).
 2. Network filter according to claim 1, whereby the filter capacitors (3) are connected to a common star point (4) that is connected to the protective conductor (1) via an auxiliary inductance (7).
 3. Network filter according to claim 1, whereby each filter capacitor (3) is connected to the protective conductor (1) via an auxiliary inductance (7) connected in series therewith.
 4. Network filter according to claim 2, whereby the star point (4) is connected to the auxiliary inductance (7) via an auxiliary capacitor (80 connected in series therewith.
 5. Network filter according to claim 1 through 4, whereby the filter inductances (2) amount to less than 50 μH.
 6. Network filter according to claim 1 through 5, whereby the filter inductances (2) are formed by a conductor surrounded by a magnetically conductive ring.
 7. Network filter according to claim 1 through 6, whereby the filter inductances (2) are formed by a conductor surrounded by a ferrite core.
 8. Network filter according to claim 1 through 7, whereby the auxiliary inductance amounts to less than 50 μH.
 9. Network filter according to claim 1 through 9, whereby one or more further filter inductances (9) are connected in series with each filter inductance (2). 